In radio communication systems, such as Wideband Code Division Multiple Access (WCDMA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) by the Third Generation Partnership Project (3GPP), or wireless LAN and Worldwide Interoperability for Microwave Access (WiMAX) by the Institute of Electrical and Electronics Engineers (IEEE), a base station device (hereinafter also called a base station, cell, serving cell, transmitting station, transmitting device, or eNodeB) and a terminal (hereinafter also called mobile station device, mobile terminal, receiving station, mobile station, receiving device, or user equipment (UE)) are each equipped with one or multiple transmit/receive antennas, and by using multiple-input and multiple-output (MIMO) technology, for example, high-speed data transmission may be realized.
In such radio communication systems, in the case in which the base station transmits downlink data to the terminal (a transport block with respect to a downlink shared channel (DL-SCH)), a demodulation reference signal (DMRS), which is known signal between the base station and the terminal, is multiplexed and transmitted. Herein, the demodulation reference signal may also be called the user equipment-specific reference signal (UE-specific RS). Hereinafter, the demodulation reference signal will also be simply designated the reference signal.
For example, the reference signal is multiplexed with downlink data before a precoding process is applied. For this reason, by using the reference signal, the terminal is able to measure an equalized channel that included the applied precoding process as well as the channel state. In other words, the terminal is able to demodulate the downlink data without being notified of the precoding process by the base station.
At this point, the downlink data is mapped to a physical downlink shared channel (PDSCH). In other words, the reference signal is used for demodulation of the PDSCH. As another example, the reference signal is transmitted only in resource blocks (also called physical resource blocks or resources) into which the corresponding PDSCH has been mapped.
Currently, radio communication systems using a heterogeneous network deployment (HetNet) made up of a macro base station with wide coverage and remote radio heads (RRHs) with narrower coverage than the macro base station are being considered. FIG. 7 is a diagrammatic overview of a radio communication system using a heterogeneous network deployment. As illustrated in FIG. 7, a heterogeneous network is made up of a macro base station 701, an RRH 702, and an RRH 703, for example.
In FIG. 7, the macro base station 701 constructs a coverage 705, while the RRH 702 and the RRH 703 construct a coverage 706 and a coverage 707, respectively. Additionally, the macro base station 701 is connected to the RRH 702 through a link 708, and is connected to the RRH 703 through a link 709. Consequently, the macro base station 701 is able to transmit and receive data signals and control signals (control information) with the RRH 702 and the RRH 703. Herein, a wired link such as optical fiber or a wireless link using relay technology is used for the link 708 and the link 709, for example. In this case, by having some or all of the macro base station 701, the RRH 702, and the RRH 703 use the same resources, it is possible to improve the spectral efficiency (transmission capacity) within the area of the coverage 705.
Also, if a terminal 704 is positioned within the coverage 706, the terminal 704 is able to conduct single-cell communication with the RRH 702. Also, if the terminal 704 is positioned near the boundary (cell edge) of the coverage 706, countermeasures against same-channel interference from the macro base station 701 become necessary. Currently, for multi-cell communication (cooperative communication) between the macro base station 701 and the RRH 702, a method that reduces or suppresses interference with respect to the terminal 704 in a cell edge area by conducting base-to-base cooperative communication in which neighboring base stations cooperate with each other is being investigated. For example, schemes such as the cooperative multipoint (CoMP) transmission scheme are being investigated as a scheme to reduce or suppress interference with base-to-base cooperative communication (NPL 1).